Has a castle ever fallen to mercenary's, commoners, or rouge knights that weren't backed by a foreign duke or king? Specifically Medieval Europe?
The Swedish peasant revolts that happened frequently during the Kalman Union från 1434 and onwards.
In fact, the revolters led by Engelbrekt destroyed many castles in Sweden during the revolt, killing our ousting the noble tax collectors that were residing there.
Agneholm castle, Köping castle, Ekeholm castle, Öresten castle, Opensten castle, Rumlaborg castle, Kindahus castle, Loholmen castle and many, many more were destroyed by the angry peasant hosts.